1. Field of the Invention
The present invention relates to a protection device, especially, the protection device that protects a current amplifier circuit against abnormal state of overcurrent.
2. Description of the Related Art
When abnormal state that output short circuit current flows into a current amplifier circuit of a power amplifier occurs, it is necessary to protect the current amplifier circuit by cutting output current quickly. Secure means for cutting the output current is a relay device. The relay device has delay time by mechanism operation. When input signal of the current amplifier circuit is high-speed, cutting by the relay device is delay in some cases. Accordingly, when area of safe operation of the current amplifier circuit is dropped to low-speed side in accordance with so-called recovery time that is from the time when supply of driving voltage is cut to the time when contacts are separated and connection is cut, the area of safe operation with a single pulse of output element is not used effectively.
In JP2559568 B, acircuit that protects an amplifier against abnormal state of excessive output current when turning on is disclosed. The circuit includes a generation unit that operates by trigger pulse and outputs turn off command signal for predetermined time that is started from timing that abnormal state of the amplifier occurs, a detection circuit that detects abnormal state of the amplifier, an OR circuit that performs OR operation of a turn off command signal and a turn on signal, and an AND circuit that performs AND operation of output of the OR circuit and output of the detection circuit. The amplifier is turned off by an OR circuit that performs OR operation of output of the AND circuit and the turn off command signal.
In JP H09-266412 A, a protection device of an element for electric power amplification is disclosed. The protection device sets predetermined area of safe operation based on polygonal line that is approximated to the maximum allowable electric power loss curve of the element for electric power amplification. The protection device includes a transistor that limits base current of the element for electric power amplification based on voltage that is generated at both ends of a resistor that detects collector current of the element for electric amplification. Herein, the maximum allowable electric power loss curve of the element for electric power amplification is a curve at DC operation. In the invention that is described in JP H09-266412 A, although larger current can be flown with a single pulse, protection is performed.
According to method of JP 2559568 B, circulation state that abnormal state is fixed when an abnormality detection signal is detected and the amplifier is turned off and the amplifier is back to the abnormal state again when the amplifier is turned on can be prevented. However, the area of safe operation is used effectively at single pulse operation and the amplifier is protected is not described at all. According to the method of JP H09-266412 A, an area that cannot be operated within the area of safe operation with DC of the element for electric amplification can be less as less as possible. However, operation of the element for electric power amplification in wide range from a DC area to a single pulse area cannot be protected effectively.